1. Field of the Invention
The present invention relates to a wireless communication device, a wireless communication method, and a computer program including a plurality of antennas and performing calibration processing to compensate for the imbalance among antenna branches, particularly to a wireless communication device, a wireless communication method, and a computer program for performing calibration processing by looping back a calibration signal between antenna branches.
More in detail, the present invention relates to a wireless communication device, wireless communication method, and a computer program for performing calibration processing to guarantee that transmitting analog circuits and receiving analog circuits of the respective antenna branches have a constant amplitude, particularly to a wireless communication device, wireless communication method, and a computer program for performing gain calibration processing to make transmitting analog circuits and receiving analog circuits of the respective antenna branches have a constant amplitude by the use of a small number of loopback paths.
2. Description of the Related Art
As a system releasing users from wiring according to an existing wired communication method, a wireless network has been drawing attention. Normal standards relating to the wireless network include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 and IEEE 802.15. The IEEE 802.11a/g standard supports a modulation method achieving a maximum communication speed of 54 Mbps. A next-generation wireless LAN (Local Area Network) standard capable of achieving a higher bit rate has been sought.
Wireless communication technologies achieving high-throughput wireless data transmission include multi-antenna technology, according to which a communication device includes a plurality of antennas. As an example of the multi-antenna technology, an adaptive array antenna is in wide use. This is a method of controlling the gains of respective antenna elements to obtain appropriate antenna directivity in transmission and reception and support communication. That is, signals received by the respective antenna elements are multiplied by respective appropriate weighting factors for weighted synthesis, and the reception directivity pattern of the entire array antenna is controlled. Further, respective transmitting signals are multiplied by appropriate weighting factors for the respective antenna elements, and are transmitted from the respective antenna elements. Thereby, the transmission directivity pattern of the entire array antenna is controlled. The array antenna method includes a sector antenna-like method in which a main lobe is directed only in a desired direction and a low side lobe is directed in an undesired direction to prevent unnecessary radio wave radiation, and a method in which a main lobe is directed in the direction of a desired mobile station and a null is directed in the direction of a mobile station acting as an interfering station to improve the SINR (Signal-to-Interference-plus-Noise power Ratio).
Further, as another example of the wireless communication technology using the multi-antenna, MIMO (Multi-Input Multi-Output) communication has been drawing attention. The MIMO achieves higher-quality communication by performing beamforming between a transmitter (beamformer) and a receiver (beamformee), each of which includes a plurality of antenna elements. The beamforming herein refers to a method of digitally weighting respective transmitting antennas and changing the antenna directivity to achieve high-quality reception by the receiver. The transmitting antenna weighting can be obtained through the analysis of a channel matrix H in the forward direction from the transmitter to the receiver. The MIMO communication method can achieve an increase in the communication speed by increasing the transmission capacity in accordance with the number of antennas, without increasing the frequency band. Further, the method uses spatial multiplexing, and thus improves the frequency use efficiency. The MIMO is a communication method using the channel characteristic, and is different from the transmitting and receiving adaptive array. For example, IEEE 802.11n, which is an extended standard of IEEE 802.11, employs the MIMO communication method.
In any of the multi-antenna technologies, there is an issue of variation in characteristic among transmitting and receiving antenna branches (characteristic of the space between one antenna branch and another antenna branch). That is, there is an issue in that, in the transmission of an RF (Radio Frequency) signal through an RF transmitting circuit or an RF receiving circuit, the influence of individual differences of active devices and components forming the circuit, such as an amplifier and frequency converters (an up-converter used in the transmission and a down-converter used in the reception), appears as the imbalance in phase and amplitude among antenna branches. Particularly, individual differences of an AGC (Automatic Gain Control) circuit in the RF receiving circuit and a PA (Power Amplifier) in the RF transmitting circuit have a significant influence.
The method of correcting the variation in characteristic among transmitting and receiving antenna branches can be roughly divided into “antenna calibration” and “IQ (In-phase, Quadrature) calibration.”
The phase and amplitude characteristics of an analog circuit included in each of the antenna branches are referred to as a “transfer function.” The antenna calibration corresponds to adjustment for maintaining a constant ratio between the transfer function of a transmitting analog circuit and the transfer function of a receiving analog circuit in each of the branches. The imbalance in transfer function among branches leads to the deterioration of the antenna characteristic in the adaptive array, and directivity different from the expected directivity is formed. Further, in the MIMO communication, the imbalance in phase and amplitude among branches leads to false channel recognition, and prevents the acquisition of an appropriate transmission beamforming matrix. As a result, the decoding characteristic of the receiver is significantly deteriorated.
For example, a proposal has been made of a wireless communication device which calculates accurate antenna calibration coefficients for respective branches on the basis of forward loopback transfer functions of paths for transmitting a known calibration signal from a reference branch, which is one of the transmitting and receiving branches, to the other branches, and backward loopback transfer functions of paths looped back from the other branches and having the known calibration signal received by the reference branch (see Japanese Unexamined Patent Application Publication No. 2007-116489, for example).
Meanwhile, the IQ calibration intends to correct an IQ amplitude error attributed to the variation in amplitude of I (In-phase) channel signals and Q (Quadrature) channel signals in an IQ modulator of the up-converter and an IQ demodulator of the down-converter, and to correct an IQ phase error indicating a shift of the I-axis and the Q-axis from the angle of 90°. If an IQ error formed by the IQ amplitude error and the IQ phase error is not corrected, the EVM (Error Vector Magnitude) of a transmitted signal and a received signal is deteriorated. As a result, the communication quality is deteriorated.
In the past, neither one of the antenna calibration and the IQ calibration has provided the effect of equalizing the amplitudes of the respective antenna branches.
The antenna calibration basically intends to maintain a constant ratio between the transmission analog transfer function and the reception analog transfer function in each of the antennas. Therefore, the antenna calibration does not satisfy the following two conditions at all.
First Condition: The amplitudes of the transmitting analog circuits of the respective antenna branches are constant.
Second Condition: The amplitudes of the receiving analog circuits of the respective antenna branches are constant.
Further, the IQ calibration adjusts the I-channel amplitude and the Q-channel amplitude to be equal in one antenna, but does not adjust the amplitudes of the respective antennas.
Herein, consideration will be given to an issue arising when the transmitting analog circuits and the receiving analog circuits of the respective antenna branches do not have a constant amplitude, i.e., when the first and second conditions described above are not met.
If the first condition is not met, i.e., if the amplitudes of the transmitting analog circuits of the respective antenna branches are not constant, waste occurs in the determination of the transmission power. For example, if the standard specifies the transmission with a power of 0 dBm (1 mW), and if there is variation in amplitude among the transmitting antennas of a communication device, i.e., if there is variation in transmission power, the transmission power is set to be lower than 0 dBm (1 mW) to meet the standard in consideration of the variation among devices. As a result, a power loss occurs, and communication with another party is prevented when the transmitted signal is supposed to reach the party.
Further, if any one of the first and second conditions is not met, the use of normal AGC or the like is prevented in many cases in the calibration for compensating for the IQ imbalance, due to the loopback in a device. In such a case, a calibration signal is monitored with the gain of the AGC (Automatic Gain Control) fixed. If the respective antennas have different amplitudes, the dynamic range is restricted due to the absence of the AGC. As a result, the number of bits used in the device (an ADC (Analog-to-Digital Converter) circuit and the subsequent stages) is increased.
In sum, it is considered significantly important to perform gain calibration satisfying the first and second conditions described above.